The present invention relates to an alarm system which includes at least one signal transmission line for providing an indication of damage to the line and/or an attempt to interfere with it, said line being suitable for incorporation in a net-structure, for example a barrier net-structure for denying foreign objects, such as underwater vessels, frogmen, and the like, access to underwater passageways, channels, etc.
Barrier net-structures intended for the aforesaid purpose are constructed to take-up extreme tensile forces. Nevertheless it is desirable that they are provided with some form of alarm system which will indicate the successful passage, e.g. of a submarine through the net, in spite of all precautions, or signal an alarm should the net be interfered with in some way. Among other things, the anchorage of the net-structure to the bed of the waterway should be protected with such an alarm system.
There has been proposed in the art an alarm facility in which a closed circuit current is passed through an electric conductor incorporated in the net. Although this arrangement functions satisfactorily enough in general, it is encumbered with a number of disadvantages. For example, the arrangement is highly sensitive, especially when used underwater, and is liable to give false alarms, due to its sensitiveness to all forms of disturbance. In addition, repairs to the net-structure require it to be lifted from the water. Another drawback is that when using large net-structures comprising several sections, or a plurality of small net-structures, the task of laying the conductor or conductors in the net(s) becomes relatively complicated. The requisite alarm centre also becomes relatively complicated since in the case of an alarm, it must be capable of indicating which net-section was responsible for the alarm being given. It is impossible to indicate the exact position in any event. Moreover, the costs and time involved in investigating such false alarms are quite considerable. Minor faults in the insulation of an electrical system must be repaired immediately, however. It is also possible to manipulate electrical systems with the aid of shunt couplings, e.g. at the shoreside connections.